In order to provide a basis for controlling human babesiosis and Lyme disease in northeastern U.S., we shall determine whether the abundance of deer limits the abundance of the deer tick (Ixodes dammini), whether increased abundance of mice may inhibit transmission of babesiosis and whether the presence of squirrels or chipmunks inhibits transmission. The role of ants as predators of deer ticks will be evaluated. These experiments in quantitative epidemiology rest on a body of biological data recently obtained in the study location. Mathematical models will be used to evaluate and describe the results of these studies.